The same assertion failure was already reported here. But unlike with those steps to reproduce BoA intro indeed triggers the problem for me using Win32 Debug configuration. Tried with WolfenDoom: Blade of Agony version 1.0 and with GZDoom at this commit.

I already had been working on a fix myself. There's one thing your PR is missing: It shouldn't completely reset the cycler if it switches from one pulse light to another one - in that case the size needs to be updated but the cycle shouldn't be reset.